Problems may be caused by water contamination in aircraft fuel systems. In particular, water contamination in fuel is known to collate in aircraft fuel tanks. When an aircraft reaches altitude during operation, the ambient temperature drops to around −50° C. which can lead to ice formation within the fuel.
The formation of ice during operation of an aircraft at altitude can lead to the reduced or ceased functionality of fuel system components (e.g. through clogging, or other processes), such as filters, fuel pumps and gauges. The presence of water in a fuel tank can also lead to the growth of microbiological contamination (MBC), which may also cause similar problems. As such it is desirable to remove water from a fuel tank before such problems occur.
A known method of removing water from a fuel system is to locate a drain valve at the sump floor of a fuel tank, or to fluidly communicate a drain valve with the sump of a fuel tank. However, such drain valves are unable to discriminate between water and fuel such that either insufficient water is drained from the fuel tank, or fuel is removed from the fuel tank together with the water. Moreover, it is necessary for an operator to manually operate a drain valve into an open flow condition, and for the drained waste fuel/water mix to be disposed of in an appropriate manner.
An alternative method for removing water from a fuel system has been proposed. WO2014/174247 describes a fuel tank for storing liquid hydrocarbon. The fuel tank has a filter fitted to the floor of the fuel tank. The filter has a permeation member such as a membrane, which is formed from a material such as graphene oxide. This membrane allows liquid water in the tank to drain out of the tank through the filter but substantially prevents the liquid hydrocarbon in the tank from doing so.
“Unimpeded Permeation of Water Through the Helium-Leak-Tight Graphene-Based Membranes”, R. R. Nair et al, Science, 27 Jan. 2012, Vol. 335, no. 6067, pp. 442-444, DOI:10.1126/science.1211694 demonstrated that sub micro meter-thick membranes made from graphene oxide can be completely impermeable to liquid, vapours, and gasses, including helium, but that the membranes allow unimpeded permeation of water.
However, a problem with a fuel tank having such a filter it is not possible to control the flow of water through the filter and out of the aircraft.